1. Field of the Invention
The present invention relates to laser scanning systems for reading bar code symbols, and more particularly, to motor control circuits for hand held portable laser scanning heads.
2. Description of the Prior Art
The increased use of bar code symbols to identify products, particularly in retail business, has resulted in the development of various bar code reading systems. Many users of bar code readers require portable hand held scanners which place a premium of size, weight and power requirements for the devices. One such system is a laser scanning bar code reading system as described in U.S. Pat. No. 4,496,831, assigned to the same assignee as the present invention and incorporated by reference herein.
The laser scanning system disclosed in U.S. Pat. No. 4,496,831 includes a portable hand held scanning head which may be embodied in various shapes but preferably has a gunshaped housing made of lightweight plastic. A handle and barrel portion are provided to house the various components of the scanning head therein. Within the barrel portion are mounted a miniature light source, a miniature optic train including focusing lenses for directing the light source across a bar code symbol, and miniature sensing means for detecting reflected light from the bar code symbol being scanned.
The miniature light source comprises a laser tube such as a co-axial helium-neon laser tube, or preferably, a semiconductor laser diode, which is considerably smaller and lighter than a laser tube, thus reducing the required size and weight of the scanning head, and making the scanning head easier to handle and more maneuverable. Light generated by the light source passes through the optic train which focuses the beam to impinge upon the scanning means, which are mounted in the light path within the barrel portion of the scanning head. The scanning means sweeps the laser beam across the bar code symbol, and comprises at least one scanning motor for sweeping the beam lengthwise across the symbol, and may comprise two motors, where the second motor sweeps the beam widthwise across the symbol. Light reflecting means such as mirrors are mounted on the motor shafts to direct the beam through the outlet port to the symbol. The sensing means then detects and processes the light reflected off the symbol, and generally comprises photosensitive elements such as semi-conductor photodiodes.
The structural aspects of the scanning motor are analogous to a simplified stepper motor which is a device used to convert electrical pulses into discrete mechanical angular movements every time the currents in the stator windings are changed. By alternately energizing and de-energizing the two stator coils of a stepper motor, the magnetic interaction between the rotor poles and the stator poles causes the rotor to turn in discrete angular steps over the entire 360 degree circumference of the output shaft. In contradistinction to stepper motors, the scanning motor is controlled by a control means which is operative to cause the shaft of the motor to oscillate first in one circumferential direction over an arc length less than 360 degrees and secondly in the opposite circumferential direction over an arc length less than 360 degrees, and thereafter to repeat the aforementioned cycle at a high rate of speed.
The motor control means for the above prior art scanning head includes a reference means for applying a generally constant low level direct current voltage to one of the stator windings, thereby energizing one set of stator poles as north and south. The motor control means also includes a variable means for applying a periodic voltage of time-varying amplitude to the second set of stator windings, thereby energizing the second set of stator poles as north and south. The first set of stator poles, due to the dc current, define a neutral rotor position. When the rotor is displaced by a small angle from the neutral position, the stator poles exert a restoring torque on the rotor which is approximately proportional to the product of the dc current and the rotor displacement. The restoring torque is very similar to that generated by a spring. The spring-like action of the dc current and the rotating mass form an oscillatory system with a natural resonant frequency which increases as the dc current increases. The second set of stator poles exert an oscillatory torque on the rotor, which is thus forced to oscillate at the same frequency of the periodic voltage. The periodic voltage is referred to as the driving voltage, the current it generates is known as the driving current, and its frequency is called the driving frequency. When the natural resonant frequency is equal to the driving frequency, a condition of resonance exists, and the driving current required to maintain a given amplitude of oscillation is at its minimum value.
In another aspect of the above referenced patent, a closed loop control circuit is disclosed to provide linear tracking of the laser scanning beam The control circuit comprises a primary coil, two or more secondary coils and a moveable shield. The shield is fixedly mounted on the motor shaft for joint oscillatory movement therewith, and is located between the primary coil and two secondary coils. Tuning capacitors are used to tune the secondary coils to resonate at the frequency at which the primary coil is excited, the secondary coils are inductively coupled to it to establish an oscillating magnetic field, an ac voltage of the same high oscillating frequency as the primary coil appears across the secondary coils, and is detected by sensing circuitry. The sensing circuitry detects the voltage on its respectively associated secondary coil, and feeds the voltages to a differential amplifier for generating a difference signal which is proportional to the angular displacement of the shaft. This difference signal is, in turn, fed to one input of another differential amplifier whose input is supplied with a control voltage. The output of the amplifier is then fed to the stator coil The aforementioned patent, therefore, sets forth an elaborate system to control the amplitude of oscillation of the motor shaft. However, the control circuit of the above referenced patent requires additional sensing circuits and apparatus to provide the feedback control signals.